The present invention relates generally to micromachined pressure transducers, and more particularly to a microphone or microspeaker comprising a cantilever structure.
Among the advantages of micromachining of pressure transducers are improved dimensional control, extreme miniaturization, the ability to integrate with on-chip circuitry, and potential low-cost as a result of batch processing.
Acoustic pressure transducers function as microphones or microspeakers. Microphones are pressure sensors that detect airborne sound pressures that are ten orders of magnitude lower than ambient pressure. Hence, a microphone needs an extremely compliant diaphragm to have an acceptable sensitivity. The diaphragm is the member that moves in response to changes in pressure.
The micromachined pressure sensors with piezoelectric readout initially had relatively thick diaphragms (on the order of tens of microns) bulk micromachined from a substrate, such as single-crystal silicon. Control of the thickness and of the latent stress in such diaphragms was inadequate for use at very thin dimensions. When use of very thin, of the order of microns, diaphragms was attempted, the resulting diaphragm was warped.
A novel process for low-stress silicon nitride thin film deposition is disclosed in U.S. Pat. No. 4,783,821, issued Nov. 8, 1988. This process made possible the fabrication of thin-film diaphragm pressure transducers. The higher compliance of the thin-film diaphragm allowed production of more sensitive microphones. These and the earlier thick diaphragms are clamped on all four edges or all four corners, resulting in a tensioned diaphragm. The tensioning is necessary to control the shape of diaphragms whose residual stresses, together with the stresses of the transducers attached to the diaphragms, tend to warp them, even in the case of the newer low stress silicon nitride films. The tension, however, decreases the compliance of the diaphragm and, as a result, the sensitivity of the microphone.
Cantilever diaphragms are much more compliant than tensioned diaphragms. Use of a cantilever would increase the sensitivity of a microphone and the intensity of the output of a microspeaker. Maximizing the effective device area, minimizing acoustic leakage (reduction of the pressure difference on the two sides of the diaphragm due to air flow around it), and controllability of the device parameters, all require fabrication of substantially flat diaphragms. This has not heretofore been possible with thin-film, cantilever diaphragm and transducer structures. Attempts to fabricate such structures resulted not only in warped diaphragms, but in many cases the residual stresses led to breakage of the diaphragm during the cantilever patterning.
Accordingly, an object of the present invention is to provide a micromachined pressure transducer having a cantilever diaphragm.
Another object of the present invention is to provide a method for fabrication of a substantially flat cantilever diaphragm and transducer with high yield.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.